Mice infected with Friend virus complex develop acute erythroblastosis, which rapidly progresses to erythroleukemia. The replication defective component of Friend virus complex, SFFV, encodes a mutant retroviral envelope protein (gp55), which confers pathogenicity. Susceptibility to Friend disease is influenced by a number of host factors. One of these, is the Friend virus susceptibility-2 gene (Fv2). Most inbred strains of mice are Fv2 sensitive, except for C57BL6 and related strains which are Fv2 resistant. Fv2 resistance does not prevent retroviral infection or replication. Rather, Fv2 appears to determine whether infected erythroblasts will proliferate in response to gp55. There is evidence that Fv2, gp55, and the erythropoietin receptor (EPOR) are part of a mitogenic complex that is constitutively active and leads to erythroblast proliferation. To test this hypothesis, and to determine the role of Fv2 in erythroid proliferation and transformation, we undertook the positional cloning of Fv2. In our preliminary experiments, we have mapped the Fv2 interval and cloned a contig of 5 bacterial artificial chromosomes (BACs) spanning that interval. We have mapped 10 genes to that interval, including the stem cell kinase receptor (STK). This receptor is a member of the scatter factor family and includes the avian oncogene, v-sea, which causes erythroblastosis in birds. We found that a truncated form of that receptor (SF-STK) is specifically not expressed in Fv2 resistant mice and that expression of SF-STK makes Fv2 resistant mice sensitive to Friend virus, in the setting of T-cell depletion. Therefore, the following experiments are proposed: In specific aim 1, we propose to determine if SF-STK is Fv2. We will do this by producing SF-STK transgenic mice on an Fv2 resistant background, and by knocking out SF-STK expression in Fv2 sensitive mice. In specific aim 2, we take a broad-based approach to identify Fv2. This includes; in vivo complementation with BACs in the Fv2 interval; generation of a transcription map of the Fv2 interval; and evaluation candidate genes molecularly, and in transgenic mice. In specific aim 3, we propose to determine the mechanism of action of Fv2, and the role of Fv2 in erythroid proliferation and transformation. We will accomplish this by knocking out STK (both forms of the transcript) in mice; with biochemical studies of the EPOR complex; and with genetic studies designed to examine the role of signal transduction pathways, downstream of Fv2 and the EPOR, in Friend virus induced proliferation. These studies should improve our understanding of normal hematopoiesis and leukemic transformation.